The message :
This ownCloud instance is currently being updated, which may take a while.
This page will refresh itself when the ownCloud instance is available again.
Contact your system administrator if this message persists or appeared unexpectedly.
Thank you for your patience.
If you have a console access, go to your owncloud directory (eg. /var/www/owncloud/)
Execute the following command :
# su -c "php occ upgrade" www-data
If you don’t have a console access, the only bad solution is to edit the configuration file config/config.php and to change the value of maintenance to false
'maintenance' => true,
'maintenance' => false,